Domestic low-level exposure to radon gas is considered a major lung-cancer hazard, involving DNA damage to bronchial cells by alpha particles from radon progeny. At low radon levels, these cells are extremely rarely traversed by more than one alpha particle, whereas at higher levels, where cancer risks are assessable (in uranium miners), cells may experience multiple alpha traversals. Measuring the oncogenic effects of exactly one alpha particle without confounding effects of multiple traversals has not hitherto been feasible, resulting in uncertainty in low-dose radon risk estimates. Charged-particle microbeams irradiate individual cells with predefined exact numbers of particles; whilst previously too slow to assess small oncogenic risks, recent microbeam developments now permit irradiation of large cell numbers, allowing the first oncogenic risk measurements for exactly one alpha particle. Oncogenicity from exactly one alpha particle was significantly lower than for a Poisson-distributed mean of one alpha, implying that cells traversed by multiple alpha particles contribute most of the risk. If this applies generally, extrapolation from high-level radon risks (involving multiple alpha particles) overestimates low level (involving only single alpha particles) risks.